# Dictionary:Fig S-12

FIG. S-12.

Snell's law relations when a wave crosses a boundary between two isotropic solid media; the wave changes direction such that for either incident P-wave or S-wave,

where *i* is the angle of the incident wave with a velocity *V*_{i}=*V*_{P1} if a P-wave or *V*_{i}=*V*_{S1} if an S-wave; and are the angles of reflection of the P- or S-waves in medium 1, which have velocities *V*_{P1} and *V*_{S1}, respectively; and are the angles of refraction of the P- and S-waves in medium 2 which have velocities *V*_{P2} and *V*_{S2}, respectively; *p* is the raypath parameter (which will be a constant along a raypath through parallel layering). If or exceeds 1 as given by this equation, a head wave will be generated.